In the fabrication of self-aligned, double polysilicon (sometimes referred to herein as "poly") bipolar junction transistor (hereinafter "BJT") structures, the polysilicon from which the base conductor is formed is typically doped with boron by ion implantation. The connection between the extrinsic base and the intrinsic base region is formed by a lateral diffusion of this boron from the base conductor into the underlying silicon. The conductor for contact to the extrinsic base is formed by the base polysilicon. In order to achieve a very low resistance base connection and contact, the polysilicon must be very heavily doped. Boron doses on the order of 5.times.10.sup.15 cm.sup.2 to 1.times.10.sup.16 cm.sup.2 are typically used.
Since the base polysilicon must be kept relatively thin in order to reduce vertical base contact resistance and to create low emitter contact structure aspect ratios, and since the boron implant must be completely contained within the base polysilicon layer to provide a repeatable link-up diffusion and shallow extrinsic base junction, the energy of the boron implant must be kept relatively low. This means that a relatively costly implant step is required. The implant step is costly due to the low throughput that results from low energy boron implants at the required high dosage concentrations.
In a double poly self aligned bipolar junction transistor (DPSA BJT) in a BiCMOS implementation, a single polysilicon is typically used for both the gate of MOS devices and the base polysilicon in the BJT. As a result, separate patterning and implant steps are used to dope the base polysilicon and the gate poly for these devices. A representative section view of a prior art BJT is shown in FIG. 1.
What is needed is a method for eliminating the need for separate patterning and implant steps in a BiCMOS process in the formation of the base polysilicon of the BJT and the gate of the MOS device.
It is with the foregoing problems in mind that the instant invention was developed.